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rsta.royalsocietypublishing.org Research Cite this article: Anderson RF, Barker S, Fleisher M, Gersonde R, Goldstein SL, Kuhn G, Mortyn PG, Pahnke K, Sachs JP. 2014 Biological response to millennial variability of dust and nutrient supply in the Subantarctic South Atlantic Ocean. Phil. Trans. R. Soc. A 372: 20130054. http://dx.doi.org/10.1098/rsta.2013.0054 One contribution of 12 to a Theo Murphy Meeting Issue ‘New models and observations of the Southern Ocean, its role in global climate and the carbon cycle’. Subject Areas: biogeochemistry, oceanography Keywords: Southern Ocean, dust, iron, biological productivity Author for correspondence: Robert F. Anderson e-mail: [email protected] Biological response to millennial variability of dust and nutrient supply in the Subantarctic South Atlantic Ocean Robert F. Anderson 1,2 , Stephen Barker 3 , Martin Fleisher 1 , Rainer Gersonde 4 , Steven L. Goldstein 1,2 , Gerhard Kuhn 4 , P. Graham Mortyn 5 , Katharina Pahnke 6 and Julian P. Sachs 7 1 Lamont-Doherty Earth Observatory, Columbia University, PO Box 1000, Palisades, NY 10964, USA 2 Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA 3 School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK 4 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany 5 Institute of Environmental Science and Technology (ICTA), and Department of Geography, Universitat Autònoma de Barcelona (UAB), Edifici Cn, Campus UAB, Bellaterra 08193, Spain 6 Max Planck Research Group, Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany 7 School of Oceanography, University of Washington, Seattle, WA 98195, USA Fluxes of lithogenic material and fluxes of three palaeo-productivity proxies (organic carbon, biogenic opal and alkenones) over the past 100 000 years were determined using the 230 Th-normalization method in three sediment cores from the Subantarctic South Atlantic Ocean. Features in the lithogenic flux record of each core correspond to similar features in the record of dust deposition in the EPICA Dome C ice 2014 The Author(s) Published by the Royal Society. All rights reserved. on July 3, 2018 http://rsta.royalsocietypublishing.org/ Downloaded from

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ResearchCite this article: Anderson RF, Barker S,Fleisher M, Gersonde R, Goldstein SL, Kuhn G,Mortyn PG, Pahnke K, Sachs JP. 2014 Biologicalresponse to millennial variability of dust andnutrient supply in the Subantarctic SouthAtlantic Ocean. Phil. Trans. R. Soc. A 372:20130054.http://dx.doi.org/10.1098/rsta.2013.0054

One contribution of 12 to a Theo MurphyMeeting Issue ‘Newmodels and observationsof the Southern Ocean, its role in globalclimate and the carbon cycle’.

Subject Areas:biogeochemistry, oceanography

Keywords:Southern Ocean, dust, iron,biological productivity

Author for correspondence:Robert F. Andersone-mail: [email protected]

Biological response tomillennial variability of dustand nutrient supply in theSubantarctic SouthAtlantic OceanRobert F. Anderson1,2, Stephen Barker3,

Martin Fleisher1, Rainer Gersonde4,

Steven L. Goldstein1,2, Gerhard Kuhn4,

P. GrahamMortyn5, Katharina Pahnke6

and Julian P. Sachs7

1Lamont-Doherty Earth Observatory, Columbia University,PO Box 1000, Palisades, NY 10964, USA2Department of Earth and Environmental Sciences,Columbia University, New York, NY 10027, USA3School of Earth and Ocean Sciences, Cardiff University,Cardiff CF10 3AT, UK4Alfred Wegener Institute, Helmholtz Centre for Polar and MarineResearch, Am Alten Hafen 26, 27568 Bremerhaven, Germany5Institute of Environmental Science and Technology (ICTA), andDepartment of Geography, Universitat Autònoma de Barcelona(UAB), Edifici Cn, Campus UAB, Bellaterra 08193, Spain6Max Planck Research Group, Institute for Chemistry and Biology ofthe Marine Environment (ICBM), University of Oldenburg,Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany7School of Oceanography, University of Washington, Seattle,WA 98195, USA

Fluxes of lithogenic material and fluxes of threepalaeo-productivity proxies (organic carbon, biogenicopal and alkenones) over the past 100 000 years weredetermined using the 230Th-normalization methodin three sediment cores from the Subantarctic SouthAtlantic Ocean. Features in the lithogenic flux recordof each core correspond to similar features in therecord of dust deposition in the EPICA Dome C ice

2014 The Author(s) Published by the Royal Society. All rights reserved.

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core. Biogenic fluxes correlate with lithogenic fluxes in each sediment core. Our preferredinterpretation is that South American dust, most probably from Patagonia, constitutes amajor source of lithogenic material in Subantarctic South Atlantic sediments, and that pastbiological productivity in this region responded to variability in the supply of dust, probablydue to biologically available iron carried by the dust. Greater nutrient supply as well asgreater nutrient utilization (stimulated by dust) contributed to Subantarctic productivityduring cold periods, in contrast to the region south of the Antarctic Polar Front (APF), wherereduced nutrient supply during cold periods was the principal factor limiting productivity.The anti-phased patterns of productivity on opposite sides of the APF point to shifts inthe physical supply of nutrients and to dust as cofactors regulating productivity in theSouthern Ocean.

1. IntroductionEstablishing the contribution by the ocean’s biological pump to past changes in the globalcarbon cycle, especially the climate-related changes in atmospheric CO2 concentration [1], is along-sought goal of palaeoclimate research (e.g. [2–4]). Biological utilization of nutrients in theSouthern Ocean is particularly important in this regard as it regulates the preformed nutrientinventory for most of the deep ocean and, therefore, the global average efficiency of the biologicalpump [4,5]. Nutrient utilization is inefficient in the Southern Ocean today, in part becausephytoplankton growth is limited by the scarcity of iron (e.g. [6]).

Martin [7] suggested that iron limitation in the Southern Ocean may have been relieved duringthe Pleistocene ice ages by the greater deposition of continental mineral aerosols (dust) thatwas discovered in Antarctic ice cores. Dust would have supplied iron to marine phytoplankton.However, early tests of this hypothesis using geochemical indicators of biological productivityextracted from marine sediment cores concluded that the region south of the Antarctic PolarFront (APF) was characterized by lower productivity during the ice ages [8–10], contrary tothe expectations from Martin’s hypothesis. The principal exception to this view was derivedby examining the species assemblages of diatoms preserved in glacial-age sediments, whichsuggested greater rather than lower ice-age productivity [11]. Despite this one dissenting finding,the common view is that the increase in dust supply did not enhance biological productivity southof the APF (for a recent synthesis, see [12]).

In contrast to the Antarctic zone, south of the APF, sediment records from Subantarctic sites(defined here as the zone north of the APF, extending as far as the Subtropical Convergence)revealed ice-age enhancement of biological productivity in all sectors of the Southern Ocean[12,13]. This is particularly true in the South Atlantic, where high levels of productivity areinferred from sediments deposited during the last ice age [14]. Initial studies concluded that ice-age productivity in the Subantarctic South Atlantic was stimulated by dust that originated inPatagonia [8], immediately ‘upwind’ of the region and known to have been the primary sourceof dust in Antarctic ice cores (e.g. [15,16]). However, this view was challenged by a number ofstudies concluding that most of the lithogenic material in South Atlantic sediments is deliveredby ocean currents rather than via the atmosphere [17–27], both during ice ages and duringinterglacial periods.

Martinez-Garcia et al. [28,29] resurrected the notion that biological productivity in theSubantarctic South Atlantic is stimulated by dust in finding a tight coupling between dustand productivity over the past 4 Myr. Specifically, at ODP Site 1090 (42.913◦ S, 8.898◦ E, 3702 m,figure 1) the accumulation rates of iron and of n-alkanes (organic compounds derived fromleaf waxes of land plants) were correlated with one another across glacial–interglacial cycles,as well as with geochemical indicators of biological productivity. Each of these parameters inODP 1090 sediments was further correlated with dust deposition in the EPICA Dome C (EDC)ice core. The tight coupling between fluxes of lithogenic material and of leaf waxes, together with

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Figure 1. Location ofmarine sediment cores discussed in this paper plotted onmaps ofmean annual concentration of (a) nitrate[NO3] and (b) silicic acid [Si] in surface waters. Mapswere prepared using Ocean Data View [30] using data from theWorld oceanatlas 2009 [31].

the correlation with EDC dust fluxes, supports the view that most of the lithogenic material inSubantarctic South Atlantic sediments is supplied as dust [8].

Although the findings of Martinez-Garcia and co-workers are compelling, correlations acrossglacial–interglacial cycles may be misleading. Many variables correlate with one another over100 kyr Milankovitch cycles simply because the enormous variability of climate boundaryconditions influences these variables, without any inherent causal connection among them.Consequently, here we expand on earlier work by examining South Atlantic sediment recordsat much greater temporal resolution, thus affording examination of dust–productivity linkagesin the absence of major changes in climate boundary condition (e.g. glacial–interglacial cycles),instead focusing on more subtle changes in forcing at higher frequency. We find a tight couplingbetween geochemical indicators of biological productivity and the accumulation of lithogenicmaterial in three cores from the Subantarctic South Atlantic Ocean, and we further demonstratethat these features exhibit a close correspondence to dust deposition in the EDC ice core onmillennial time scales. We conclude that phytoplankton in the region responded to increased dustsupply with an increase in nutrient utilization and increased export production.

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2. Material and methodsAccumulation rates of sedimentary constituents were evaluated for three South Atlantic cores(figure 1) using the 230Th-normalization approach [32]. This approach makes a necessarycorrection for the redistribution of sediments by deep-sea currents (sediment focusing), whichcan cause the local accumulation of sediment to exceed the regional average particle rainrate by as much as a factor of 20 in the cores studied here [33]. Concentrations of lithogenicmaterial were estimated for each core using the measured 232Th concentration. Concentrationsof Th isotopes (230Th and 232Th) were measured by isotope dilution using methods describedelsewhere [34,35].

Thorium-232 abundance has a fairly narrow range in upper continental crust [36,37], andamong dust sources worldwide [38]. Here, we use an average content of 10 ppm to estimatethe lithogenic component of each sample, acknowledging that there is a potential uncertainty ofas much as 20% in this estimate depending on the source of the lithogenic material. Assessingthe temporal variability of lithogenic supply is more important here than the absolute flux,so we accept the uncertainty in the absolute flux inherent in this approach in favour of theprecision that it offers for detecting small changes in the lithogenic flux over time. We make noinitial assumption about the origin of the lithogenic material. Undoubtedly, for each site somefraction is delivered by ocean currents as well as by dust. We will use the amplitude of thetemporal variability that correlates with dust deposition in the EDC ice core [39] to provide semi-quantitative constraints for the fraction of the lithogenic component of sediments that consistsof dust.

Mackensen et al. [40] presented an initial estimate of palaeoproductivity for core PS2498-1(44.15◦ S, 14.49◦ W, 3783 m). In addition to thorium (see above), we add to the study of PS2498-1new measurements of organic carbon (C-org, determined at the Alfred Wegener Institute (AWI)),using an elemental analyser. Biogenic opal was also measured, both at AWI using the method ofMüller & Schneider [41] and at the Lamont-Doherty Earth Observatory (LDEO) using the methodof Mortlock & Froelich [42]. Opal concentrations measured at LDEO were systematically greaterthan those measured at AWI, most probably reflecting a difference in the efficiency of Si extraction[43]. Despite the systematic offset, the downcore pattern of opal content derived using the twomethods was internally consistent. As for the lithogenic fluxes, the precision with which we canassess temporal variability is more important for our objectives than the accuracy of the opalflux, so the opal results from LDEO are used without implying any judgement about the relativeaccuracy of the two datasets.

Concentrations of C37 alkenones, a suite of organic compounds produced by coccolithophorids,and thorium have been described previously [33] for TN057-06PC4 (42.91◦ S, 8.9◦ E, 3751 m) andTN057-21-PC2 (41.13◦ S, 7.81◦ E, 4981 m). Here, we report the summed concentration of the C37methyl ketones with two (C37:2Me) and three (C37:3Me) double bonds, and refer to that quantitysimply as ‘alkenones’. Although more than 10 different alkenones and the related alkenoateshave been reported in marine sediments, C37:2Me and C37:3Me are the two compounds used forpalaeotemperature reconstructions and are by far the most abundant alkenones in Subantarcticsediments. Those results are supplemented here with additional measurements of thorium forboth cores.

3. Age modelsLithogenic fluxes in PS2498-1, on the age model of Mackensen et al. [40] (figure 2b), share a numberof features with the EDC dust record (figure 2a); for example, elevated fluxes during the intervalsapproximately 18–30 ka and approximately 60–70 ka, as well as three shorter intervals of elevatedflux between 30 and 60 ka. Given the similarity of the records, we selected a number of tie points toalign lithogenic fluxes in PS2498-1 with EDC dust fluxes (solid black lines in figure 2). In addition,the age model was modified using radiocarbon ages reported by Gersonde et al. [44] for the core

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Figure 2. (a) Flux of dust in the EPICA Dome C (EDC) ice core [39]. (b) Flux of lithogenic sediment in core PS2498-1 estimatedfrom 230Th-normalized fluxes of 232Th (see text for details) on the original agemodel of Mackensen et al. [40]. Age control pointsused to adjust the age model of PS2498-1 by aligning the lithogenic flux with the EDC dust flux are shown as solid black lines.(c) Flux of lithogenic sediment in core TN057-06 estimated from 230Th-normalized fluxes of 232Th on the agemodel of C. Charles(2000, personal communication). Age control points used to adjust the age model of TN057-06 by aligning the lithogenic fluxwith the EDC dust flux are shown as dashed grey lines.

top down to a depth of 151 cm (age of 22 ka). The original age–depth relationship for PS2498-1 is compared with the revised age model in figure 3. Oxygen isotope data for PS2498-1 [40]placed on the adjusted age model fitted the stacked record of Lisiecki et al. [45], the benchmarkfor Pleistocene chronology of deep-sea sediments, at least as well as when using the original agemodel (not shown, but available from the lead author).

For TN057-06PC4, we began with the age model of Hodell et al. [46] as modified by C. Charles(2000, personal communication). Further adjustments to the age model were made to align thelithogenic fluxes of TN057-06-PC4 with dust deposition in the EDC record (figure 2, dashed greylines). As for PS2498-1, the oxygen isotope record for the adjusted age model fitted the Lisieckistack at least as well as with the original. The revised age–depth relationships for PS2498-1 andTN057-06 exhibit less variability over time than with the original age models (figure 3). While amore uniform accumulation rate does not necessarily mean that an age model has been improved,the internal consistency among all results following the adjustments to these age models (see alsobelow) supports the use of the new age models.

Several age models have been published for TN057-21-PC2. Building upon previous work[47,48], we first tied features in the TN057-21 record to features in the oxygen isotope record of theNorth GRIP ice core. The absolute age of both records beyond 60 ka was then adjusted by aligningthe North GRIP oxygen isotope record with the northern Alps (NALPS) speleothem record [49],which has absolute age control from U–Th dating [50]. Adjustments to the age model of the EDCice core were then made to place it on the NALPS chronology as well, maintaining the relationshipbetween EDC and North GRIP developed by previous studies. While further adjustments to theabsolute age of the records may be necessary, more important is that all of the marine and icecore records used here have been tied to a common age model to facilitate intercomparison of thevarious results.

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4. ResultsLithogenic fluxes in the three Subantarctic cores are compared with the EDC dust flux record infigure 4. Fluxes of opal and of C-org in PS2498-1, as well as the lithogenic flux, are shown with theEDC dust record in figure 5. Alkenone fluxes from TN057-06 and from TN057-21 are presentedin figure 6, together with the lithogenic fluxes from each core, to compare with the EDC dustrecord. Alkenone concentrations are also shown in figure 6d to show that variability of the 230Th-normalized alkenone flux is determined mainly by variability in the concentration of alkenonesrather than by variability of 230Th-normalized mass flux.

5. Discussion

(a) Sources of lithogenic materialCores PS2498-1 and TN057-06 were recovered from sites on the eastern flank of the Mid-AtlanticRidge [40] and on the Agulhas Ridge [46], respectively. Their location on elevated topographyand their distance from continental margins isolates these sites from turbidites and from contourcurrents delivering sediment from continental sources. Therefore, one can have confidence thatlithogenic sediment delivered to these sites was transported either via the atmosphere or by majorocean currents, such as the Antarctic Circumpolar Current, allowing for minor contributions frommaterial transported by icebergs.

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Figure 4. (a) Flux of dust in the EPICA Dome C ice core [39]. Fluxes of lithogenic sediment in Subantarctic South Atlantic cores(b) PS2498-1, (c) TN057-06 and (d) TN057-21 estimated from 230Th-normalized fluxes of 232Th (see text for details). Age modelsfor PS2498-1 and TN057-06 were tuned to the EDC dust record as illustrated in figures 2 and 3. The age model for TN057-21 wasderived by tuning to the North GRIP ice core (see text) and, therefore, it is independent of the EDC dust record.

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Figure 5. (a) Flux of dust in the EPICADomeC ice core [39]. Fluxes of (b) lithogenic sediment, (c) organic carbon and (d) biogenicopal in Subantarctic South Atlantic core PS2498-1. Fluxes of sediment constituents were estimated by 230Th normalization (seetext for details).

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Figure 6. (a) Flux of dust in the EPICA Dome C ice core [39]. (b) Flux of lithogenic sediment in TN057-21. (c) Flux of lithogenicsediment in TN057-06. (d) Concentration of alkenones in TN057-21 from [33]. (e) Flux of alkenones in TN057-06 from [51]supplemented by new 230Th data. (f ) Flux of alkenones in TN057-21. Records from TN057-21 are in grey to help distinguishbetween the two TN057 sediment cores. Fluxes of sediment constituents are estimated by 230Th normalization (see textfor details).

Lithogenic fluxes in PS2498-1 and in TN057-06 (figure 4b,c) exhibit a close correspondenceto the pattern of dust deposition in the EDC ice core (figure 4a), including two major intervalsof elevated dust flux (approx. 18–35 ka and 60–70 ka), two shorter but intense episodes of dustdeposition centred at approximately 41 ka and at approximately 50 ka, as well as smaller features(e.g. 55–60 ka and approx. 89 ka). The observed correspondence is consistent with an aeoliansource for a large fraction of the lithogenic material in PS2498-1 and TN057-06. Furthermore,during intervals of maximum lithogenic flux, fluxes are nearly twice as large on the Mid-AtlanticRidge (PS2498-1) as on the Agulhas Ridge (TN057-06), in agreement with previous observationsof an eastward decrease in lithogenic flux across the Subantarctic South Atlantic during the LastGlacial Maximum (LGM) [8], and consistent with expectations, as dust is washed out of theatmosphere while transported downwind from a South American source.

In contrast to the other cores, TN057-21 was recovered from a drift deposit in the deepCape Basin where contour currents deposit sediment entrained along the margin of Africa[52]. Mineralogical [22] and isotopic [25] composition of lithogenic material at this locationare consistent with an African source for a portion of the sediment, with an additional time-varying source of material originating at higher latitudes [22]. This combination of two majorsources of lithogenic material at the site of TN057-21 creates a record of lithogenic accumulation(figure 4d) that has a smaller amplitude of variability compared with the other cores (figure 4b,c).

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Building on the prior work of Kuhn & Diekmann [22], we interpret the record of lithogenicaccumulation in TN057-21 to reflect a time-varying aeolian component, similar to the recordin TN057-06, superimposed on a relatively constant source of sediment from Africa. Despiteits reduced amplitude of variability, the record of lithogenic flux in TN057-21 contains mostof the features that correspond to the EDC dust record, including some of the smaller peaks(e.g. approx. 30 ka).

Our results do not permit us to quantify precisely the fraction of lithogenic material inthe marine cores that was delivered as dust. Future studies involving geochemical provenancetracers will help in this regard. Nevertheless, based on our finding that the principal features inthe lithogenic flux record correspond to features in the EDC dust record, we conclude that themajority of the lithogenic material in PS2498-1 and in TN057-06, as well as a large fraction oflithogenic material in TN057-21, was delivered by atmospheric transport, most probably fromSouth America.

(b) Biological response to dustBiogenic constituents of the sediments are interpreted as indicators of biological production.Accumulation of biogenic material in marine sediments is influenced by preservation as wellas by production, and preservation is difficult to quantify. Therefore, rather than attempt toquantify preservation, we employed two biogenic tracers (opal and organic carbon) for whichpreservation is not expected to covary. Preservation of opal is sensitive to temperature, but inthe cold stable environment of the abyssal ocean the preservation of opal depends mainly onsediment accumulation rate [53]. Preservation of organic carbon, on the other hand, is mostsensitive to the concentration of oxygen in bottom water [54], which is not expected to covarywith sediment accumulation rate. Therefore, covariability of opal and of C-org is interpreted toreflect changes in production rather than variable preservation.

Accumulation rates of C-org (figure 5c) and of opal (figure 5d) in PS2498-1 are correlatedwith one another, as well as with the accumulation rate of lithogenic material (figure 5b) andwith EDC dust deposition (figure 5a). For the eastern sites (TN057 cores), we use alkenones,long-chained ketones produced exclusively by certain haptophyte algae, predominantly thecoccolithophorids Emiliana huxleyi and Gephyrocapsa oceanica in the open ocean [55,56], as apalaeoproductivity indicator (figure 6). Alkenones were also employed as a palaeoproductivityindicator by Martinez-Garcia et al. [28,29] in their study of ODP1090 (figure 1). Features in TN057-06 are less well defined because of the roughly fivefold lower accumulation rate in this corecompared with TN057-21, which allows for greater smoothing of the record by bioturbation.Nevertheless, despite the differing depositional regimes (pelagic deposition on a topographicalelevation versus deep drift deposit), the principal features of the TN057-06 alkenone record(figure 6e) are consistent with those in TN057-21 (figure 6d,f , [33]), supporting the validity ofthese tracers as indicators of time-varying changes in biological productivity.

Palaeoproductivity proxy records in each Subantarctic core exhibit features that covary withthe lithogenic flux in the same sediment core, even over millennial time scales (figures 5 and 6).Tight coupling over short time intervals lessens the potential influence of changes in globalclimate boundary conditions, thereby strengthening the view that the productivity record reflectsa biological response to varying supply of dust, and to the iron that it carries [8,28]. However,factors other than dust may also regulate biological productivity in the Southern Ocean, and theseare discussed in the next section.

Before proceeding, we want to reiterate that, whereas the age models for PS2498-1 andTN057-06 were adjusted by tuning to the EDC dust record, the age model for TN057-21 wastuned only by alignment with the oxygen isotope record of the North GRIP ice core [49].Therefore, the correspondence of the palaeoproductivity record of TN057-21 (figure 6d,f ) withthe records from the other cores (figures 5 and 6), as well as with the EDC dust record(figure 6a), supports the reliability of tuning the age models of PS2498-1 and TN057-06 to the EDCdust record.

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–0.2

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Figure 7. Concentration of alkenones (grey) and δ13C of planktonic foraminifera G. bulloides (black) in TN057-21. Alkenoneconcentrations are from [33] and carbon isotope data are from [59].

(c) Supply and utilization of nutrientsThe Subantarctic zone is a region of persistently high nitrate concentrations (figure 1a; phosphate,not shown, has a similar distribution). Therefore, the greater fluxes of organic material duringperiods of elevated dust flux, which correspond to the colder intervals in Antarctica [39], couldreflect a more efficient utilization of nitrate due to relief from iron limitation, or a greatersupply of nutrients, or both. Nitrogen isotopes in foraminifera-bound organic matter fromODP1090/TN057-06 have been interpreted recently to indicate greater nitrate utilization efficiencyduring periods of elevated dust supply [57], providing compelling evidence for iron fertilization.

Although the results presented here are consistent with a strong sensitivity of biologicalproductivity to dust supply in the Subantarctic South Atlantic Ocean, dust supply cannot be thesole factor regulating productivity. For example, all of the core sites are located in regions wheresurface waters today are impoverished in dissolved silicon (Si, figure 1b). At the site of PS2498-1, the flux of opal during the late glacial period (20–35 ka; 0.25–0.30 g cm−2 kyr−1) was aboutsixfold greater than during the last 10 kyr (approx. 0.05 g cm−2 kyr−1; figure 5d). Nearly identicalresults were reported previously for a site further to the east (PS2082-2, 43◦13.2′ S, 11◦44.3′ E, [9]).Substantially greater opal fluxes during the LGM are recorded at sites further south, but still wellto the north of the APF (approx. 50◦ S in this region), ranging between 0.6 g cm−2 kyr−1 (PS1754-1/-2, 46◦46.2′ S, 7◦36.7′ E, [9]) and 1.6 g cm−2 kyr−1 (RC15-93, 46◦06′ S, 13◦13′ W, [8]). Opal fluxesthis large require a source of Si much greater than exists today to sustain the diatom productivityimplicated by the opal flux.

A greater supply of nutrients to the Subantarctic zone during glacial periods has also beeninferred from the carbon isotopic composition (δ13C) of planktonic foraminifera in TN057-21[58,59]. Many of the features in the alkenone record of TN057-21 are correlated with the δ13C ofGlobigerina bulloides in this core, in the sense that increasing productivity corresponds to increasingconcentrations of nutrients (decreasing δ13C; figure 7). However, δ13C of planktonic foraminiferais sensitive to other factors in addition to nutrient concentration (e.g. [60,61]), so a nutrient–productivity correlation cannot be inferred unambiguously. Furthermore, over two intervals(approx. 65–60 ka and approx. 18–10 ka), the palaeoproductivity indicator drops dramaticallywhile the δ13C remains low (consistent with, but not necessarily requiring, high nutrients).Over these intervals, the palaeoproductivity indicator correlates much better with dust supply(figures 5 and 6).

On the other hand, there are occasions when this correlation breaks down. In TN057-21, forexample, alkenone concentrations are relatively high during times of low or declining lithogenicflux at approximately 39, 48 and 57 ka (figure 6). In PS2498-1, elevated fluxes of opal and ofC-org also persist until approximately 39 ka, despite declining dust flux (figure 5). Each of theseintervals, corresponding to Heinrich Stadials 4, 5 and 5A, coincides with low δ13C in G. bulloides(figure 7), which we interpret to indicate that nutrient supply may play at least as great a roleas dust flux in regulating productivity at these times. Enhanced upwelling south of the APF

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during these intervals [62] may have injected nutrients into the Subantarctic zone [51] as wellas in the Antarctic zone. We conclude, therefore, that both increased supply of nutrients andincreased nutrient utilization efficiency, stimulated by dust, contributed to the rise in Subantarcticproductivity during cold periods, including relatively cold intervals of millennial duration duringMarine Isotope Stage (MIS) 3 (approx. 30–60 ka) and late MIS 5 (prior to approx. 80 ka) as well asthroughout MIS 2 and 4.

(d) Contrasting conditions across the Antarctic Polar FrontSubantarctic productivity can now be considered in the context of a substantial body of workdescribing patterns of productivity throughout the entire Southern Ocean. In contrast to modern(Holocene) conditions, when export production south of the APF exceeds that of the Subantarcticzone, the situation was reversed during the LGM, when export production of the Subantarcticzone was much greater than to the south of the APF (e.g. [8–10]). Kohfeld et al. [12] provide acomprehensive synthesis of the evidence for the last glacial period, whereas Jaccard et al. [63]show that the seesaw pattern of productivity across the APF was a regular response to climatevariability throughout the late Pleistocene.

Two general scenarios have been proposed to explain lower export production of the Antarcticzone during glacial periods. First, expansion of sea ice during glacial periods [64] may havelimited the growth of phytoplankton in the Antarctic zone (e.g. [10,65]). Implicit in this scenariois the corollary that a greater fraction of the nutrients upwelled south of the APF would remainunused and thus be subjected to Ekman transport northward into the Subantarctic zone. Thatis, the greater supply of nutrients to the Subantarctic zone during glacial periods could simplyreflect lower utilization in the Antarctic zone superimposed on conditions of upwelling andsurface water transport similar to those that exist today. The principal alternative scenario is thatnutrient supply by upwelling south of the APF was reduced during glacial periods (e.g. [66]),most probably tied to increased stratification of the polar ocean (for reviews, see [67,68]).

Nitrogen isotopes archived in Southern Ocean sediments have been interpreted to favourthe second hypothesis (reduced nutrient supply). Throughout the Antarctic zone, the nitrogenisotopic composition of organic compounds preserved within diatom frustules indicates greaternitrate utilization during glacial periods, coincident with lower export production, comparedwith interglacials [69–71]. Relatively efficient utilization of nitrate during glacial periods isinconsistent with inhibition of phytoplankton growth by sea ice, but it allows for iron fertilizationin response to the greater supply of dust. That is, the lower rate of nutrient supply during glacialperiods trumps the effect of iron fertilization, reducing export production within the Antarcticzone compared with iron-limited interglacial periods.

The combination of lower supply and greater utilization of nitrate south of the APF duringglacial periods eliminates the northward transport of unused nutrients as the primary source tofuel Subantarctic productivity. Instead, during glacial periods, nutrients must have been suppliedto the Subantarctic zone by mixing from below.

Today, the zone of maximum upwelling and nutrient supply is located to the south of theAPF [72]. Two scenarios can be envisioned to account for the northward displacement of nutrientsupply during glacial periods. First, the APF may have migrated northward by several degrees oflatitude (reviewed by Gersonde et al. [64]), carrying the band of upwelling with it into latitudesmore favourable for phytoplankton growth. Alternatively, the hydrographic features associatedwith the APF may have remained locked roughly in their present position through interactionwith bottom topography [73]. Under this scenario, a northward shift of the southern westerlies[74] may have decoupled the zone of upwelling from the APF. Upwelling, driven primarilyby wind stress curl in the Subantarctic zone under these conditions, may have ventilatedmuch shallower layers of the deep ocean than occurs today. While this scenario is speculative,ventilation of shallower water masses in the Southern Ocean during glacial periods is consistentwith a growing body of evidence for greater mid-depth stratification during the LGM [75,76]. Of

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course, the two scenarios are not mutually exclusive, as the westerlies and the frontal systemsmay have migrated concurrently.

Lastly, we note that the seesaw pattern of nutrient supply across the APF that is evident overglacial–interglacial cycles is also apparent over much shorter time scales, with some exceptions.Anderson et al. [62] interpreted opal fluxes to indicate increased upwelling south of the APFduring the most intense Northern Hemisphere stadials of the last glacial cycle, which correspondto relatively warm Antarctic Isotope Maxima (AIM). Dust fluxes to Antarctica are at relativeminima during AIM events, and peak during the coldest intervals [39]. Therefore, if the lithogenicfluxes to Subantarctic sites correspond to dust fluxes in Antarctica, then the covariance betweendust, export production and nutrient supply (figures 5–7) indicates maximum supply of nutrientsto the Subantarctic zone during Antarctic cold intervals, opposite to the pattern observed south ofthe APF. The principal exceptions, noted in the previous section, occur during Heinrich Stadials4, 5 and 5A, when nutrient supply to the Subantarctic zone remained high during the transitioninto Antarctic warm intervals. Records with greater temporal resolution will be needed to resolvethe precise phasing between temperature, dust, nutrients and productivity at these times.

6. Summary and outlookAccumulation rates of lithogenic material in three sediment cores from the Subantarctic SouthAtlantic Ocean exhibit features over the past 100 000 years that correspond to the pattern of dustdeposition in the EPICA Dome C ice core, indicating that most of the lithogenic material in thesediments was derived from South American dust sources. Accumulation rates of biogenic opaland of organic indicators of biological productivity also correlate with the lithogenic fluxes inthese cores. Combining these results with evidence for nutrient utilization [57] and for nutrientsupply, we conclude that export production in the Subantarctic zone was stimulated during coldperiods by the synergistic effects of greater nutrient supply together with increased nutrientutilization, supported by elevated dust fluxes.

Nutrient supply and export production south of the APF varied in a pattern that was anti-phased with their variability in the Subantarctic zone, whereas nutrient utilization was high inboth regions during the LGM and, presumably, during earlier cold intervals as well. Conditionssouth of the APF indicate that export production during the LGM was limited more by the supplyof macronutrients than by iron. The anti-phased pattern of nutrient supply on opposite sides ofthe APF, combined with the dust fluxes presented here, further indicates that the physical forcingthat brings nutrients to the surface varied almost concurrently with dust supply.

Climate-related shifts in the latitude of the Southern Hemisphere westerlies have beensuggested to influence nutrient supply in the Subantarctic South Atlantic [47,77] as well as inthe Antarctic zone [62]. Meridional shifts in the southern westerlies may also have modulated thegrowth and retreat of mountain glaciers in the mid-latitudes of the Southern Hemisphere [78–81],which, in turn, may have regulated the source of glaciogenic sediments entrained by the windsas they passed over Patagonia [82]. Thus, a north–south displacement of the southern westerliesprovides a unifying hypothesis that is consistent with the palaeoclimate records described earlier.Future studies should consider why these features do not appear to be prominent in modelsimulations (for recent discussion, see [83,84]).

Acknowledgements. Helpful comments from Wally Broecker, Chris Charles, Joerg Schaefer, Gisela Winckler,Aaron Putnam and Ben Bostick are much appreciated. Insightful reviews from two anonymous refereessubstantially improved the paper. Opal data from PS2498-1 were generated by Patricia Malone, who passedaway while this manuscript was under review. Her many years of support for the palaeoceanographycommunity at Lamont are much appreciated. She will be dearly missed by many friends and colleagues.Data accessibility. Results presented here that have not been published previously will be submitted to thePANGAEA (http://pangaea.de/) and NCDC (http://www.ncdc.noaa.gov/) data archives. Data from corePS2498-1 are available at PANGAEA DOI via the link: http://doi.pangaea.de/10.1594/PANGAEA.832084.DOIs for data from cores TN057-21 and TN057-06 are not yet available.

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Funding statement. New results reported here were generated with support from the US NSF via award OCE0823507.

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